We are assessing the character and extent of experimental myocardial ischemia. We have developed a fluorophotographic technique that permits high resolution (plus or minus 50 microns) examination of the redox state of intramitochondrial pyridine nucleotide. In an isolated, crystalloid perfused rabbit heart we believe the distance between perfused tissue and myocardial ischemia ("borderzone") is approximately one quarter of a millimeter. In normoxic, blood perfused systems, the borderzone may be even narrower. We also have incriminated the peri-ischemic borderzone as the origin of ventricular tachyarrhythmias in both dogs and man. We are currently investigating patterns of ventricular activation in both animals and man. We have also located the origin of these arrhythmias in the peri-ischemic borderzone in man and surgically excised this zone in 33 patients.